GWD5-Q34请教

GWD5-Q34:

Many entomologists say that campaigns to eradicate the fire ant in the United States have failed because the chemicals that were used were effective only in wiping out the ant’s natural enemies, which made it easier for them to spread.

1. which made it easier for them
           
2. which makes it easier for it
           
3. thus making it easier for them
           
4. thus making it easier for the ant
5. thereby, it was made easier for the ant

我选的E,

谁能帮忙解释下，c,d里making it easier，it所指吗？

我是因为觉得it指代不明，E里面it是形式主语才选的。

请指教。谢谢！

首先，thereby是副词，不是名词，后面不能跟一句完整的句子；

CD中的it是形式宾语，真正的宾语是to spread，整句话的意思是making (to spread) easier

GWD5-Q34:

Many entomologists say that campaigns to eradicate the fire ant in the United States have failed because the chemicals that were used were effective only in wiping out the ant’s natural enemies, which made it easier for them to spread.

1. which made it easier for them
               （which 指代enemies，错误，them 不能指代the fire ant）
2. which makes it easier for it
               （which 同上，两个it指代对象不一致，前者指代to spread，后者指代the fire ant）
3. thus making it easier for them
               （them 指代错误）
4. thus making it easier for the ant （正确， making 做伴随状语，表示结果，修饰that 后面的句子，不知道这里有没有修饰主句的嫌疑...盼NN指导一下！）
5. thereby, it was made easier for the ant（thereby是副词，不能做连词引导一个句子，构成run-on sentence， was made easier 结构不好）

欢迎讨论！

不是NN，

应该没有修饰主句嫌疑吧，可以理解为修饰the chemicals吧。我也盼NN能路过一下。